I. Technical Field
This invention pertains to the field of solenoids, and particularly to magnetic latching solenoids.
II. Related Art and Other Considerations
A typical solenoid has a moveable member which is connected to or integral with a plunger or piston. The moveable piston or plunger, which can be in the form of an output shaft, is the serving or working element/aspect of the solenoid that can be employed in any of various applications or utilizations.
One type of solenoid is a “power stroking” or “power on” solenoid. In a natural state of a power stroking solenoid, the solenoid moveable member is separated by an air gap from a solenoid stationary member. The solenoid also has a coil or the like which, when energized, creates a magnetic flux. The magnetic flux generated by the coil results in the moveable member being electromagnetically attracted to the stationary member(s). Depending on the positioning and configuration of the piston relative to the moveable member, attraction of the moveable member toward the stationary member can cause the piston to be retracted or extended relative to its original position. The moveable member is held in place (in attraction) to the stationary member until power is removed from the coil. When power is removed, the moveable member returns to its original separated position (e.g., the moveable member is again separated from the stationary member by an air gap). Return of the moveable member to its original position is often facilitated by a spring or the like. An example power stroking solenoid which operates generally in accordance with the foregoing but with piston extension upon power stroking is shown in U.S. Pat. No. 4,812,884 to Mohler, entitled “Three-Dimensional Double Air Gap High Speed Solenoid”, which is incorporated herein by reference.
In contrast to a power stroking solenoid, a “holding” solenoid starts with a minimal air gap between the moveable member and the stationary member. When the holding solenoid is powered (e.g. by energization of a solenoid coil), the electromagnetic attractive forces hold the moveable member rigidly to the stationary member.
A magnetic latching or “maglatch solenoid” is a derivative of the “holding solenoid” and further includes an internally compressed spring and a permanent magnet. In its natural (and unpowered) state, the moveable member is magnetically latched to the stationary member while compressing the spring. When powered, the permanent magnet's holding force is reduced sufficiently that the spring can force the moveable member away from the stationary member.
Thus, a magnetic latching solenoid typically comprises a coil, a spring, a permanent magnet, and at least two metal components that provide a magnetic path for the magnet's flux. The spring is located between the two metal components, one of which contains the permanent magnet. As the one metal component moves toward the other, the spring is compressed. When the metal parts are brought within close proximity of each other, they latch together since the magnetic attracting force between the two metal components is greater than the opposing mechanical spring force. To unlatch (release) a magnetically latched solenoid, current (power) is applied to the coil housed within the metal components. This release power provides sufficient magnetic flux to offset/cancel the permanent magnet's flux, such that the spring force is now greater than the magnetic attracting force between the two metal components. With the magnetic attracting force thus overcome, the metal components separate (unlatch). Applications for this type of solenoid include circuit breakers, door locks, brake locks, etc.
As the moving metal component is re-latched to its mating stationary metal component during repeated actuations, variations in the magnetic circuit and air gaps between the metal components of typical magnetic latching solenoids result in release power variations that are unacceptable to the customer. Release power is the power (current and voltage) applied to the coil that allows the moveable member to be released from the stationary member. The release power variations can result in piston action that is non-uniform (e.g., with respect to one or more of piston position/placement, piston actuation power, or piston speed/response).
Since air gaps reduce magnetic efficiency when latched, a “zero” air gap magnetic latching solenoid is optimal. The location and size of air gaps, e.g., gaps between the moveable member and the stationary member, significantly affect the solenoid's performance. Even the smallest air gap is deleterious to the electromagnetic flux fields and flux paths which travel through the stationary member and the moveable member. Although a zero air gap is not yet achievable with contemporary designs, the air gap should be kept as small as possible.